Leelah explained that she always felt like a girl and wanted her parents to accept that. They did not, mother Carla Alcorn told CNN, for religious reasons. Though she loved her child, the mother struggled to wrap her mind around what transgender means.

"The only way I will rest in peace is if one day transgender people aren't treated the way I was, they're treated like humans, with valid feelings and human rights," Leelah pleaded in her note. "My death needs to be counted."

There is more information than ever about transgender people -- from new research to a freshly written book of personal essays, "Trans Bodies, Trans Selves" to the critically acclaimed new Amazon series "Transparent."

What is transgender?

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Transgender is an umbrella term for people whose gender identity, expression or behavior does not conform to the sex they were assigned at birth, according to American Psychological Association, which recently posted a Q&A on the topic. Sex is different from gender: Sex refers to genitalia, and gender is the cultural construct society has created to differentiate between men and women.

Transgender also gets confused with sexual orientation. Just because someone is transgender doesn't mean they are gay, lesbian or bisexual.

It's also important to know the difference between transgender people and transsexuals. Transsexuals alter or want to change their bodies by using hormones, surgery or other means to come more in line with the gender with which they associate, the association says. Not all transgender people want to change their bodies.

Although it may seem like a relatively new topic, transgender people have been documented since antiquity in many cultures around the world.

A transgender person has told you they are transgender. What's the first thing you should say?

Some experts say you should try this: "I love you and support you."

That first reaction from a friend or family member whose judgment matters is critical, said Aidan Key, the founder of the family education and support organization Gender Diversity in Washington. "Consider that they're facing many other challenges, and you can offer a place where they can simply be accepted, a place without struggle."

Chances are very good it's taken a tremendous amount of courage for your friend or family member to come to you.

Elizabeth Crankshaw, who was born a male but identifies as a woman, said it's also OK to "say you don't understand, because how could you? You'd have to be trans to get it."

Crankshaw grew up in the 1970s when there was no commonly parsed language about being transgender. She was married and came out first to her ex-wife and then to her sister.

"It was terrifying. I had so much anxiety," she recalled. "You have to think of the worst things that could happen. How will my employer will react? Will I lose my job? Will my co-workers treat me differently?"

If a transgender person is renting a home, she added, they could be afraid of backlash from a landlord.

If you feel the urge to ask a transgender person whether they've physically changed their genitalia, think twice.

There's no steadfast rule on this, say transgender people and experts who study gender and sexual identity issues. This is where sensitivity and some common sense come in, and generally it's best to not ask this question. Put yourself in their shoes. Would you want them to ask you about your body's most private parts? As Matt Kailey writes on his highly read blog, Tranifesto, transgender people should be afforded the same respect as anyone. Their bodies are their own.

"Trans people are not public property," he writes. "Touching something on a person to see if it is 'real' or asking personal questions about a person's body or sex life is inappropriate -- unless the person has invited you to 'ask me anything.' Otherwise, do not do or say anything that you would not do or say to anyone else."

Is it allright to ask, 'When did you know you were transgender?'

Don't ask this question. Instead, try directing the question at yourself, advises Crankshaw. When did you know you were a girl or a boy? Can you pinpoint that as a choice you made, or did it happen subconsciously?

"Transgender people knew just the same as you," said Key. "The difference is that we didn't have the language to describe it."

The pronoun conundrum: What is appropriate?

Most transgender people want to be referred to by the pronoun of the gender with which they identify. "This is the shortest, simplest way to say, 'I see you; I acknowledge you,' " Key said.

If you don't always get it right, don't freak out. Don't make big deal of it, Kailey writes on Tranifesto.

When Ryan Cassata, a transgender 21-year-old, told his mother years ago that he felt like a boy rather than a girl, she immediately showed him love and acceptance. But it took practice for Fran Cassata to call him the new name -- Ryan -- that he wanted.

She laughs, recalling the experience. "I kept using 'she' and then I finally said to him, 'Well, you try to call me by a different name. Don't call me Mom anymore. You can call me 'Your Highness.' "

Mother and son navigated a very difficult experience with humor, a method she encourages.

How do you parent a child who identifies as transgender?

Ryan Cassata is now a singer who performs and travels around the country giving speeches about his experience. There are usually parents in the audience who are just beginning to go through what he and his mom did years ago.

"I understand that it can feel really hard," Fran Cassata said. "But I ask them to take some time to think about whether it is really, really that hard."

At first, parents might go through a mourning period because they are grieving the loss of a son or daughter -- the child's assigned gender.

"For me it didn't last very long because I realized Ryan was still the same person," she said. "Your child is still your child. They still have the same laugh; their favorite color and foods are the same. It's all the same love. It's just now your daughter is your son or your son is your daughter."

Experts also point to the Family Acceptance Project at San Francisco State University, which offers support and a wealth of information to help keep families talking and together. Clinical social worker Caitlin Ryan directs the project, which in the past 14 years has trained more than 60,000 families, mental health providers and clergy in the United States, China and some Latin American countries.

Could it be a phase?

Key works with a lot of families and children, especially elementary school students who are just beginning to express a gender that is different from the one assigned to them at birth. Some parents will be inclined to ask, "Is this a phase?"

She advises parents to give themselves permission to think of it as a phase if it helps them.

"What if it is a phase? Take that question further," she said. "Don't we as parents want our children to be able to explore and learn?"

A parent should be concerned with sending an immediate message to their child that they accept them.

The phase question is tied, of course, to "What if my child changes his or her mind?"

"Well, OK, then they do!" said Key. "Give them that freedom."

Do you think your religious beliefs don't allow you to accept transgender people?

This is a far more complex discussion than can be had in this story. But Key has some insight. He was raised in a fundamentalist Christian household. His mother happened to adore him, he said, and he always felt accepted. Leelah Alcorn said in her suicide note that she felt rejection from her Christian parents. A transgender person may also struggle when it comes to religion: If they believe, they may fear that their God will ultimately reject them.

Key works with families of various religious beliefs who have a transgender relative.

"I always tell them to start with the core values of a religion: love and acceptance and nonjudgment," said Key. "There may be an inclination to go talk to a pastor or some other leader."

Rethink doing that, he said.

A person who is not the child's parent probably isn't going to have the same bond or the same level of concern, said Key. And it could feel like a scary betrayal to a child if you go to another adult that they've not consented to knowing about their transgender identity. Believe in your connection with your child, and that will resonate with him or her.

How hard can it be to navigate this world as a transgender person?

Beyond emotional challenges and questions of self, a transgender person has to live in a society that is defined by "male" and "female." In 2013, for example, CNN wrote about a transgender first-grader who was born a boy but identified as a girl.

The child's parents won a long legal battle for their child to use the girls' restroom at her Colorado elementary school.

The Colorado Rights Division said that keeping Coy Mathis, born with male genitalia, from using the girls' bathroom created "an environment that is objectively and subjectively hostile, intimidating or offensive."

It was a first-of-its-kind ruling in the country regarding transgender students' rights.

That sounded like progress to Fran Cassata.

When Ryan Cassata was in school less than a decade ago, he was told he could not use the boys' or girls' bathroom but had to go to a nurse's station to sign in, his mother said. He always signed the name "Ryan." One day the nurse told him that he couldn't go to the bathroom unless he signed his birth name, Fran Cassata recounted. So the teen went to principal, who told the nurse that the teenager could say he was Superman, just let him take care of business.

"You hope there are kind people your child will encounter," she said. "You hope there are people who help them rather than ridicule them."

But there will be people like that nurse. There will be people far worse.

"Just be a parent," she said. "Trust that they can handle themselves, and give them your strength when they need you."